cmd/go: fix incorrect determination of import path is DirImportPath

In practice this only shows up when a vendored package, imported on the
command line, imports an internal package.

Change-Id: I34c161d1f1ef15a87c58a422f17d11f77fbac53f
Reviewed-on: https://go-review.googlesource.com/c/go/+/439735
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/modload/load.go

@@ -753,6 +753,7 @@ func (mms *MainModuleSet) DirImportPath(ctx context.Context, dir string) (path s
 				suffix := filepath.ToSlash(dir[len(modRoot):])
 				if strings.HasPrefix(suffix, "/vendor/") {
 					longestPrefixPath = strings.TrimPrefix(suffix, "/vendor/")
+					continue
 				}
 				longestPrefixPath = mms.PathPrefix(v) + suffix
 			}

src/cmd/go/testdata/script/vendor_internal.txt

@@ -0,0 +1,16 @@
+go build ./vendor/foo.com/internal/bar/a
+
+-- go.mod --
+module example.com/x
+go 1.19
+
+require "foo.com/internal/bar" v1.0.0
+-- vendor/modules.txt --
+# foo.com/internal/bar v1.0.0
+## explicit
+foo.com/internal/bar/a
+-- vendor/foo.com/internal/bar/a/a.go --
+package a
+import _ "foo.com/internal/bar/b"
+-- vendor/foo.com/internal/bar/b/b.go --
+package b